Pressing Ctrl+S or F8 will freeze the game in-game, which are the keys to save in the Map Editor. Easily replicable and will always happen, custom or online games.
EDIT: Even if it's replicable, I wouldn't reccomend doing it, since it corrupts the map, crashes the game and forces you to redownload it. [Thanks Lunk]
K I D A E S T H E T I C
Would you like to partake in the fine activity known to many, especially Fred, as "yiffing"?
Try my map Ventjorn! PM me Ksenia pictures.Green is for healing